The present invention relates to an optical head device in which the signal recording, reproduction, erasion or the like for a disk-like optical information recording medium or magneto-optical information recording medium (hereinafter referred to as optical disk for simplicity) is made using a plurality of light spots. More particularly, there is an optical head in which a plurality of light spots including, for example, a recording light spot and a reproducing light spot are used together. In a certain type of such optical head, laser beams having different wavelengths are respectively used for the plurality of light spots, for example, with first and second laser beams of different wavelengths .lambda..sub.1 and .lambda..sub.2 being used for the recording and reproducing light spots, and those laser beams can be detected separately from each other with respect to the wavelengths. The present invention relates to such an optical head device using a plurality of light spots.
As an optical head using a plurality of light spots having different wavelengths, there has hitherto been known an optical head in which one objective lens is used commonly to the plurality of light spots and the light spots are converged on an optical disk by the one or same objective lens, as disclosed by, for example,
In such a conventional optical head, only one specified light spot, that is, one of two light spots actually used is subjected to an irradiation position control (or focusing control) for focusing a light beam as a light spot on an optical disk to irradiate the optical disk with the light spot while the other of the two light spots is caused to follow the one light spot, thereby simplifying an optical system and a mechanical system for focusing control. In this connection, there are the case where two light spots are both maintained in focused conditions and the case where one of two light spots is maintained in a focused condition and the other light spot is maintained in a defocused condition as required.
More particularly explaining, the focusing control is made in regard to a certain specified light spot in such a manner that a focusing error is detected from a light beam of the specified light spot reflected from the optical disk and the distance of the objective lens relative to the optical disk is controlled so that the focusing error becomes zero. Since the objective lens thus controlled is used for the other light spot too, there results in that the other light spot follows the specified light spot subjected to the focusing control though the other light spot itself is not directly subjected to the focusing control.
In such a conventional optical head, it is required that all of light spots having different wavelengths should be converged on the same recording surface of an optical disk by use of a common objective lens. For that purpose, an achromatic lens free of a change in focal distance attendant upon a change in wavelength or a chromatic aberration is used as the objective lens.
However, such an achromatic objective lens is designed on the premise that a substrate of the optical disk irradiated with a light beam through the achromatic objective lens is made of a specified material (for example, glass). An optical disk is a recording medium which is composed of a transparent substrate and a recording film provided on the surface of the substrate. A light beam is converged on a recording surface of the recording film through the transparent substrate. Therefore, if the substrate material of an optical disk actually used is different from a specified substrate material used upon design of the achromatic objective lens, there results in that the characteristic of a change in refractive index for different wavelengths of incident light beams, for example, .lambda..sub.1 and .lambda..sub.2 (hereinafter referred to as refractive index versus wavelength characteristic for simplicity) is different between the actually used substrate and the specified substrate used upon design.
Accordingly, even if the distance of the objective lens relative the optical disk is controlled by subjecting a light spot of wavelength .lambda..sub.1 to a focusing control in the case where an optical disk made of a substrate material other than a specified material (for example, glass) is used, there does not result in that a light spot of wavelength .lambda..sub.2 follows the light spot of wavelength .lambda..sub.1 subjected to the focusing control. Namely, the focus positions of the light spots of wavelengths .lambda..sub.1 and .lambda..sub.2 (or the converging position thereof in the case where it is required that the light spot should be maintained in a defocused condition) deviate from each other in the direction of an optical axis.
Now consider the case where an achromatic objective lens designed to converge light spots of different wavelengths in the same recording plane is used for a disk substrate of thickness t having a refractive index versus wavelength characteristic in which the refractive index is n.sub.1 for light of wavelength .lambda..sub.1 and n.sub.2 for light of wavelength .lambda..sub.2. If the refractive index of a disk substrate actually used is (n.sub.1 +.DELTA.n.sub.1 ) for light of wavelength .lambda..sub.1 and (n.sub.2 +.DELTA.n.sub.2) for light of wavelength .lambda..sub.2, the light spot of wavelength .lambda..sub.1 and the light spot of wavelength .lambda..sub.2 deviate from each other in focus position by a distance .delta. approximately represented by the following equation: EQU .delta.={.DELTA.n.sub.1 /n.sub.1 (n.sub.1 +.DELTA.n.sub.1)-.DELTA.n.sub.2 /n.sub.2 (n.sub.2 +.DELTA.n.sub.2)}.multidot.t (1)
By way of example, FIG. 2 shows the refractive index versus wavelength characteristics of glass and polycarbonate resin which are generally used as the substrate material of an optical disk. For example, consider the case where an achromatic objective lens is designed to exhibit the optimum achromatic characteristic when the beam wavelengths .DELTA..sub.1 and .DELTA..sub.2 of semiconductor lasers employed in a two-laser optical head using two light spots are 780 nm and 680 nm and a disk substrate material is glass. In such a case, if the achromatic objective lens thus designed is used in an optical disk using polycarbonate resin as a disk substrate material, it becomes apparent from the characteristic diagram shown in FIG. 2 and the equation (1) that a relative deviation larger than about 1 .mu.m occurs between the converging positions of the two light spots.
Therefore, when one of the two light spots is converged on a recording surface of the optical disk by a focusing control, the other light spot is remarkably defocused, thereby remarkably deteriorating the performance of the optical head. In order to avoid such a problem, it is required for the conventional optical head device using a plurality of light spots that an optical disk capable of being used should be limited to one in which a specified substrate material is used. However, this requirement provides a great problem in the aspect of generality.